I do this – mainly to drop into a lower gear approaching a steep downhill corner followed by a long downhill into a 20mph limit. It also works to smooth out the type of "drop a gear and hoof it" applications you describe!
I just blip the throttle by enough to raise the revs to the same level they will be when the lower gears go in. Difficult to describe when to do it as its become subconscious, but generally whilst the clutch is in and the gear lever on its way through the gate, but I guess I overlap with clutch in/out as well.
Done well the engine revs at the top of the blip should perfectly match the new engine speed once you let the clutch back in. This has the potential to reduce clutch wear as well as smooth out your driving.
For more info look up heel & toe gear changing and double-declutching on YouTube. Couple more bits of info – if you read the Police & Advanced driving handbook (Roadcraft) it teaches a system of car control that will reduce the need to change gears under pressure like this. Also worth knowing is that the new Nissan 370Z will do this for you automatically! As well as sounding cool, it also reduces the risk of sudden engine braking locking up the rear wheels!
Hope this helps